"Infectious disease is one of the great tragedies
of living things – the struggle for existence between different forms of
life. Man sees it from his own prejudiced point of view; but clams, oysters,
insects, fish, flowers, tobacco, potatoes, tomatoes, fruit, shrubs, trees,
have their own varieties of smallpox, measles, cancer, or tuberculosis.
Incessantly, the war goes on, without quarter or armistice – a nationalism
of species against species."(1)

A week before, Justin
Middens had lain near death with bubonic
plague in an Albuquerque hospital. He had developed a high fever and
painfully swollen lymph nodes that required an emergency evacuation from
his desert campsite where he was conducting field research on packrat behavior.
A local doctor fortunately recognized his symptoms and treated him immediately
with tetracycline. The quick action probably saved his life. According
to the Center for
Disease Control, 10 to 20 cases of bubonic plague occur in the United
States each year, mostly in northern
Arizona and New Mexico. It is still a life threatening disease, as
was evident in Justin’s case. Now, with the crisis over, he was recovering
and reading passages from the classic book on infectious disease, Rats,
Lice, and History by Hans Zinsser (1), loaned to him by a doctor. He
was puzzled by the following passage, which he read aloud to his research
advisor, Perry Daugs, who had come to see how he was doing."Resistance to infectious disease, an acquired characteristic,
is not hereditary except in the evolutionary sense of selective survival
of the more resistant...."Justin asked, "Why wasn’t I resistant to the plague? All
my ancestors came from Europe. Certainly those living in medieval times,
when about 25% of the people
in Europe died, must have survived because they were resistant."

Dr. Daugs, launched into mini lecture. "Like snowflakes,
no two humans are identical. The frequency of mutations is such that even
identical twins are unlikely to be absolutely identical genetically. Heterogeneity
in the gene pool of Homo sapiens, as with other species, provides
the raw material for rapid evolution when natural selection is strong.
As you noted, during the Middle Ages at least 25 million people, a quarter
of the population of Europe at the time, died of the Black
Death, bubonic plague (2). This is an example of strong selection for
both resistance and for an effective response. One would not be able to
detect that an evolutionary event had occurred by comparing the physical
appearance of the survivors with that of the general population before
the plague; however, selection certainly altered the gene pool and the
population was more fit with respect to plague resistance. But, over generations
in the absence of selection, independent assortment and genetic recombination
of resistance genes can lead to individuals with increased or decreased
resistance. Your experience suggests that you are lucky we have antibiotics."

For initial group discussion:
Everyone gets sick from time to time. Consider the causative agents of
the various human
infectious diseases you know and classify them. Does the human body
respond to all in the same way? How and why do the medical treatments for
each differ?

For next time: Assume
you had a time machine and were able to return to the Middle Ages with
the tools of modern molecular genetics that in principle can assay for
the presence of every allele of every gene in the human genome. If you
wished to monitor the changes in allele frequency resulting from Bubonic
Plague, what differences would you expect? Why?

Page 2: A World of Different PeoplePage 3: Warfare at the Molecular Level

References1. Zinsser, H. (1933) Rats,
Lice and History, Little, Brown and Co., Boston.2. Burnet, M. and White,
D. O. (1972) Natural
History of Infectious Disease, 4th edition, Cambridge University Press,
Cambridge, U.K.Note: Refs. 1 and 2 are
classics and are highly recommended reading for anyone aspiring to be a
physician. In particular, they show the importance of an evolutionary perspective
on disease.